1. Field of the Invention
The present invention generally relates to a power supply for a personal computer and, more particularly, to a power supply for a personal computer having a short circuit protection circuit located on a secondary side thereof.
2. Description of Related Art
The power supply for a computer system, for example, a personal computer (or "PC") converts alternating current (or "AC") power, typically from an AC input line, most commonly, a 120 volt AC main, to direct current (or "DC") power suitable for use by the various energy-demanding components of the PC. Accordingly, conventional power supplies typically include a transformer having a primary side connected to the AC input line and a secondary side connected to a DC output line. The transformer converts the AC voltage to a value which will, when rectified and filtered, provide the desired DC voltage. Accordingly, one or more rectifiers, for example a full-wave rectifying circuit, and a filter consisting of inductance in series and capacitance in shunt with the DC output line are typically provided between the secondary side of the transformer and the DC output line. Typically, the various energy-demanding components of the PC will require voltage at different levels, most commonly, .+-.12 volts, .+-.5 volts and +3.3 volts. Accordingly, power supplies are also provided with appropriate voltage dividing circuitry as well.
Conventional power supplies are further equipped with a power switch used to make or break an electrical connection with the AC input line. As the primary purpose of the power switch has been to enable the selective disconnection and reconnection of the PC with the AC input line, the power switch for the power supply has been traditionally located on the primary side. Prior designs of power supplies have avoided placing the power switch on the secondary side of the power supply, most likely for the reason that, while such a secondary side power switch would successfully disconnect the various energy-demanding devices of the PC, the power supply itself would continue to draw power from the AC input line, thereby resulting in both unnecessary consumption of electrical power and increased wear on the power supply. Furthermore, as the "powered-down" PC would still be electrically connected to the AC input line, there would be a greater likelihood that the PC would pose a safety hazard.
However, while it has been traditionally preferred that power supplies are designed such that the power switch is placed on the primary side thereof, such a placement is not without its own drawbacks. If placed on the low voltage, secondary side of the power supply, the power switch would be a relatively inexpensive item. However, when placed on the high voltage, primary side of the power supply, the cost of the power switch increases dramatically. For example, a power switch placed on the primary side of the power supply must be rated for 120 volts. Furthermore, various governmental regulatory agencies, particularly in Europe, have very strict requirements for components located across a high voltage AC line. Thus, the power switch must be designed to comply with various regulations intended to increase the safety of the switch.
Likewise, protection circuits, i.e., devices which protect the power supply from damage due to hazardous operating conditions such as short circuits or overvoltages, have been traditionally placed on the primary side of the power supply. One obstacle to placing a protection circuit on the secondary side of the power supply is that, once the protection circuit has automatically powered down the computer system, the power supply cannot be reset.
It can be readily seen from the foregoing that it would be desirable to place a protection circuit on the secondary side of the power supply which is capable of disconnecting the primary side of the transformer from the AC input line. It is, therefore, an object of the present invention to provide such a power supply having those features described herein.